On the separation of air flow over water waves

Conditions leading to the onset of air-flow separation over a mobile air-water interface are discussed. It is argued that, in a frame of reference in which the interfacial boundary assumes a steady shape, the occurrence of separation requires a stagnation point on the interface. In the case of air blowing over water waves, this corresponds to the onset of wave breaking. These arguments are strongly supported by flow visualization and pressure measurements carried out in a laboratory wind-wave flume. Furthermore, the pressure measurements show a greatly enhanced interfacial shear stress for a breaking wave compared with that over an unbroken wave of the same wavelength. The implications of these findings for wind-wave generation are discussed.